The field of the invention is in the high frequency filter art and more particularly in that of ferrimagnetic power limiters in microwave systems.
The basic principles of frequency selective powers limiters employing ferrimagnetic materials are well known. In the limiters described here, the phenomenon causes adsorption and reflection of the incident power when the level of this power is greater than a certain threshold value. If a much weaker signal is simultaneously present along with the higher power, above threshold, signal, the weak signal will not be absorbed or otherwise distorted provided that its frequency is sufficiently displaced from that of the strong signal, e.g., typically by at least a few MHz.
In limiters which are used to protect the sensitive elements in the front ends of receivers, a very low threshold power is usually desired, of the order of a milliwatt. To achieve this low threshold, various methods are used to intensify the RF magnetic field in the vicinity of the ferrimagnetic material, including the use of dielectric materials and dielectric resonators. In a typical prior art device which made use of a waveguide cavity, early investigators were able to achieve a 14 dBm power threshold in a device having a 20 MHz bandwidth. If this approach were used to achieve a 1 milliwatt (0 dBm) limiting threshold by narrowing the bandwidth, the resulting bandwidth would be so small, i.e., about 1.5 MHz, that the limiter would have few practical applications. Also, narrowing the bandwidth of the limiter to 1.5 MHz would result in increasing the insertion loss to 12 dB, making it useless as a receiver front-end protector.
The dielectric resonator approach previously mentioned requires certain minimum resonator dimensions; the size cannot be indefinitely reduced to achieve lower and lower threshold power levels. In addition, only a limited number of materials, namely the titanates, such as strontium titanate with a relative dielectric constant .epsilon..sub.r =300, and titanium dioxide, for which .epsilon..sub.r =98 are presently available. Dielectric resonator limiters which make use of these materials have several drawbacks, namely (1) variation of the dielectric constant and therefore of the resonant frequency with temperature, and (2) a tendency to crack under the mechanical stresses induced by high temperature gradients. Adverse environments may also produce similar conditions and tendencies. Finally, fabrication of dielectric resonators is a fairly expensive process.
There was a need prior to this invention, therefore, for a RF limiter structure which yielded very low threshold power while maintaining useful operating bandwidth and adequately low signal insertion loss. The present invention fulfills these requirements.
The best known prior art may be found in the following patents: U.S. Pat. No. 4,044,357 to patentee Goldie; U.S. Pat. No. 3,906,404 to patentee Dixon; U.S. Pat. No. 3,500,256 to patentees Carter et al, U.S. Pat. No. 3,289,112 to patentee Brown; U.S. Pat. No. 3,082,383 to patentee Stern; and U.S. Pat. No. 2,553,649 to patentee Garfitt.